This application is related to my copending application Ser. No. 849,584 entitled "FDM/TDM Transmultiplexer" and Ser. No. 849,271 entitled "High-Speed FFT Processor," assigned to the same assignee as the present invention.
In each of the above-mentioned applications, I have disclosed an FDM/TDM transmultiplexer which is capable of very high operating speeds while requiring very little space. The transmultiplexer is designed to convert 60-channel super groups from FDM-to-TDM and vice versa. Since, in addition to providing a transmultiplexer capable of processing 60-channel super groups, it was also my purpose to simplify the maintenance and decrease both the size requirements of the transmultiplexer and the cost of manufacturing it, a modular design format was adopted. The TDM-to-FDM conversion channel required a sixth-order elliptic filter having a 2 kHz cut-off and also required a second-order filter. The FDM-to-TDM conversion channel required only the sixth-order elliptic filter having the 2 kHz cut-off. In order to conserve size and cost, it was preferable to use in each case a single filter multiplexed over all 60 channels rather than a bank of filters, one for each channel.
Several problems were encountered in obtaining such a filter. Since each of the sixth-order filters had to be reconfigured as low-pass filters for even channels and bandpass filters for odd channels, it was necessary to use a filter capable of changing its frequency response very rapidly. In order to serve the modular design concept to both simplify maintenance and decrease production costs by enabling mass production of modular components, a single filter structure was needed which could serve as both the second-order filter and the sixth-order filter. Thus, a filter was needed which was flexible enough to be used for both filtering functions, which was also capable of high-speed real time reconfiguration and, finally, which was capable of sufficiently high operating speeds to enable multiplexing the filter structure over all 60 channels.
Known arithmetic structures for achieving digital filtering are either too slow to achieve both real time reconfiguration and multiplexing over 60 channels, or do not exhibit the required programming flexibility.
It was discovered through computer simulation that the out-of-band loss requirements of the transmultiplexer could be achieved by cascading three bi-quad arithmetic structures in order to achieve a sixth-order elliptic filter, but it was necessary to design such a structure capable of being cascaded while simultaneously being multiplexed over 60 voice channels and reconfigured in real time as a low-pass and bandpass filter for alternating channels.